Optical discs such as CD-R (Compact Disc-Recordable) and CD-RW (Compact Disc-Rewritable) typically has a laminar structure in which signals such as sounds, letters and/or images are recorded circumferentially on a discoidal substrate made of plastic such as polycarbonate or the like or a recording layer formed on the discoidal substrate, and a reflective layer is formed on the surface of the substrate or the recording layer by vapor deposition or sputtering using metal such as aluminum, gold or silver. In this case, signals are recorded or reproduced by irradiating the optical disc with a laser beam from the substrate surface side.
Recently, signal recording capacity of optical discs is significantly increased, and signal information is more highly densified as can be seen in DVD+RW, DVD-RW, DVD-RAM, because information volumes handled by use of computer memory, memory for images and sound clips, optical cards, and the like have remarkably increased. Presently, CD has a recording capacity of around 650 MB, and DVD has a recording capacity of around 4.7 GB, however, further high-densification of recording is demanded. As a way to enhance recording density in optical systems, shortening wavelengths of a semiconductor laser beam source for use and increasing numerical aperture (NA) of object lens for use are studied. Further, not only enhancement of recording density in a two-dimensional direction but also a technique of which the number of recording layers is multiplied in a thickness direction of the recording medium to store recorded information are studied.
As a means to obtain a high-capacity optical recording medium, when multiple recording layers are formed in a direction of laser beam irradiation and a laser beam with blue wavelengths is used, it brings about the following problems.
For example, in the case of an optical recording medium having two recording layers, in order to enhance the amount of light incident on a second recording layer disposed at the innermost side as viewed from the light beam irradiation side and in order to enhance the transmittance of the returned beams of light, the transmittance of light beam incident on a first recording layer disposed at the light beam irradiation side needs to be assured. To assure the light transmittance property, it is important to select materials and thicknesses of layers that are subjected to light absorption. Particularly, selecting materials of recording layers becomes an important issue. Patent Literature 1 discloses that when the thicknesses of recording layers are made thin to reduce the light absorption, the crystallization rate tends to be relatively lowered. Patent Literature 2 discloses a means to enhance the crystallization rate and employs a means to enhance the crystallization rate of a recording-layer material itself. The method for enhancing the crystallization rate of a recording-layer material itself is disclosed in Patent Literature 3. When the thickness of a recording layer is made ultrathin, the transmittance of the light beam is increased, however, the power of light beam to be absorbed at the first recording layer is reduced by the amount of transmitted light beam, and differences in recording signals being sufficient enough to read the signals are hardly obtainable. As mentioned above, there are technological difficulties in achievement of multi-layered optical recording medium.
By the way, from the perspective of materials of recording layers, there are two mainstreams of developments in the materials. Namely, one of the mainstreams is developments in GeTe which is a material used for recordable recording layers, inducing phase-changes on recording layers, and recording layer materials made of a ternary alloy of GeSbTe from a solid solution or a eutectic composition of the above-noted two recording layer materials. The other mainstream is developments in alloys of Sb and Te similarly to the above, however, this is a recording layer material made of a eutectic composition of Sb and Sb2Te3, and microelements are added to SbTe with a Sb content of around 70%.
As for the ternary alloy materials, Patent Literature 4 to 8 disclose that an addition of Ge to the main component of Te enabling reversible phase-changes can stabilize amorphous phase of Te, further, mixing Sb2Te3 with the above materials can reduce the optical energy required for recording, and by determining the mixture ratio within an optimum range, information can be recorded, erased, and rewritably recorded at high-linear velocities. Among the above mentioned documents, Patent Literature 5, 6, and 8 respectively disclose an optical recording medium having multiple recording layers.
As for the latter recording layer material of SbTe alloy, Patent Literature 9 discloses an example of a phase-change recording material of which Sb and Te are employed as the main component, and the atomic ratio satisfies the condition of 2.3<Sb/Te≦4.0. The invention describes that since the phase-change recording material has a high-crystallization rate, it enables recording, reproducing, and rewriting at high-transmission rate with stability.
However, with recording materials which are made of a compound existing on an extended line of GeTe and/or Sb2Te3, which are shown in Patent Literature 5, 6, and 8 stated above, each of the elements has a high-melting point and a high-crystallization temperature, and thus the rewriting speed is not sufficiently fast. In order to improve the recording linear velocity, a laminar structure in which a crystallization supporting layer made of a metal alloy is formed on and under a recording layer just like in Patent Literature 10, and a laminar structure in which an interface layer made of GeN or the like have been generally used. Interface layers made of GeN or the like are disclosed in Patent Literature 2 and Non-Patent Literature 1. The layers used for improving recording linear velocity in the documents can be said as a negative factor for first recording layers for which light transmittance property is required, because the layers absorb not a little amount of light. Thus, for materials of a first recording layer, a material that enables recording without a high-power, and forming with a simple laminar structure is desired. Further, since compounds existing on an extended line of GeTe and/or Sb2Te3 respectively have a low C/N ratio (Carrier to Noise Ratio) of 30 dB, it is disadvantageous in that a stable system is hardly structured when rewritable optical disc system which is said to require at least 45 dB is structured.
Next, as reflective layers of optical recording media, metal or alloy is used. For example, as disclosed in Patent Literature 11, Ag or Ag alloy is often used for reflective layers. For the reason, Ag or Ag alloy is excellent in thermal conductivity and in light reflection property. However, Ag monomers have a problem with the storage stability, and there is a need to additionally form a barrier layer or an intermediate layer for preventing Ag from reacting with other substances. Since carbide or metal is used for materials of barrier layers or intermediate layers, absorption of optical energy by a reflective layer, a barrier layer and an intermediate layer disadvantageously affects a laminar structure having multiple recording layers in terms of effective utilization of optical energy.
As for inventive examples relating to materials of Ag reflective layers, there are proposals of improving the trade-off relationship between durability and reflectance of Ag reflective layers by use of additives to assure the durability and reflectance, besides the use of Ag alloy (see Patent Literature 12).
As for examples relating to materials of reflective layers other than Ag or Ag alloy, there are reflective layers using Cu (see Patent Literature 13 to 16), and using Au (see Patent Literature 17).
However, Patent Literature 14 stated above relates to reproducing only ROM (Read Only Memory) media and is unrelated to recordable optical recording media. Patent Literature 15 stated above employs Cu for the reflective layer, however, only AgPdCu alloy is shown in the examples, and there is no specific description on a reflective layer using Cu as the main component. In addition, Patent Literature 16 describes that Cu is used as the main component for the reflective layer, however, in fact, the invention discloses a reflective layer made of Ag as the main component, and there is no specific description on Cu.
In addition, Patent Literature 18 discloses an optical recording medium having multi-layered recording layers and described that Cu is mainly used for the metallic reflective layer, and the metallic reflective layer has a thickness of 2 nm to 10 nm, however, in fact, there are only examples of reflective layers using only Cu in the examples of the invention, and there is no description on employment of other additional components for the metallic reflective layer. In the examples, it is disclosed that the storage reliability can be ensured with only the use of Cu, without the necessity of using other additional components. According to our findings, however, the storage property with a single use of Cu turned out to be degraded. Further, it turned out that even when other additional components are added, it is ineffective in keeping the storage property high depending on the type of components to be added.    Patent Literature 1 Japanese Patent Application Laid-Open (JP-A) No. 2002-144736    Patent Literature 2 Japanese Patent Application Laid-Open (JP-A) No. 2002-293025    Patent Literature 3 Japanese Patent Application Laid-Open (JP-A) No. 2003-16687    Patent Literature 4 Japanese Patent (JP-B) No. 2692654    Patent Literature 5 Japanese Patent (JP-B) No. 3216794    Patent Literature 6 Japanese Patent Application Laid-Open (JP-A) No. 10-116441    Patent Literature 7 Japanese Patent Application Publication (JP-B) No. 08-032482    Patent Literature 8 Japanese Patent Application Laid-Open (JP-A) No. 2001-143322    Patent Literature 9 Japanese Patent Application Laid-Open (JP-A) No. 2002-288876    Patent Literature 10 Japanese Patent Application Laid-Open (JP-A) No. 2002-123977    Patent Literature 11 Japanese Patent Application Laid-Open (JP-A) No. 2002-140838    Patent Literature 12 Japanese Patent Application Laid-Open (JP-A) No. 2004-39146    Patent Literature 13 International Publications No. WO/02/021524    Patent Literature 14 Japanese Patent Application Laid-Open (JP-A) No. 2003 331381 2003-338081    Patent Literature 15 Japanese Patent Application Laid-Open (JP-A) No. 2000-215516    Patent Literature 16 Japanese Patent Application Laid-Open (JP-A) No. 2002-25115    Patent Literature 17 Japanese Patent (JP-B) No. 3087433    Patent Literature 18 Japanese Patent Application Laid-Open (JP-A) No. 2005-524922    Non-Patent Literature 1 On pp. 85-90, collected lecture papers of The 10th Symposium on Phase Change Optical information Storage (1998)